Prins reaction under manganese dioxide control: the synthesis of 6-oxa-2-azabicyclo[3.2.1]octan-4-ones from tetrahydropyridines and formaldehyde

The course of the acid-catalysed Prins reaction of tetrahydropyridines 1 with formaldehyde, leading to derivatives of piperidinodioxane 4 and 3-oxa-7-azabicyclononanes 5 and 6, is dramatically changed in the presence of manganese dioxide to give new products, 6-oxa-2-azabicyclo[3.2.1]octan-4-ones 7 and 8.